1. Field of the Invention
The present invention relates to optical coupling devices in which optical coupling between an optical element such as a light-emitting element or a light-receiving element and another optical element such as an optical fiber or a lens can be realized with high efficiency and relates to manufacturing methods thereof.
2. Description of the Related Art
Referring to FIGS. 7 and 8, the case in which a laser diode (hereinafter referred to as “LD”) which is a light-emitting element is coupled with a lens will be described as an example of a related technique. As the optical coupling device described above, a technique has been well known in which a guide groove having two tapered side surfaces decreasing the distance therebetween in the depth direction is formed in a silicon (Si) substrate for alignment, and a lens is fixed in the guide groove and is then coupled with an optical element. FIG. 7 is a perspective view of an optical coupling device 80 as described above, and FIG. 8 is a cross-sectional view taken along the line 8—8 in FIG. 7 and showing the state in which a lens 35 is placed in a guide groove 33. In FIG. 8, the line X—X indicates an optical axis.
As shown in FIGS. 7 and 8, in the surface of a Si substrate 31, the guide groove 33 holding the lens 35 is formed, and on a surface of the Si substrate 31, which is in the vicinity of the end portion of the groove 33, an LD 32 is disposed. For forming the guide groove 33, anisotropic wet etching is used which can easily perform highly accurate fine etching. When a substrate having a {100} plane on a surface is used as the Si substrate 31 and is then etched by using a mask having a rectangular opening in which one side thereof is formed along the optical axis direction, due to the difference in etching rate between individual crystal planes of the Si substrate 31, the guide groove 33 having tapered side surfaces 37 and 38 decreasing the distance therebetween in the depth direction is formed. The side surfaces 37 and 38 both have {111} planes. When the lens 35 is fixed to the guide groove 33, the position of the lens 35 in the radial direction is determined so that the optical axis thereof is aligned with that of the LD 32, and as a result, the optical coupling between the LD 32 and the lens 35 can be realized.
A method for representing a crystal plane using braces { } such as a {111} plane is generally used for denoting crystallographically equivalent planes having different directions.
However, according to the technique described above, since a {111} crystal plane 34 (an inclined angle θ of 54.7°) is formed at an end portion of the guide groove 33, due to the presence of this {111} crystal plane 34, the LD 32 and the lens 35 cannot be disposed close to each other. That is, since the Si crystal is composed of a single element, only one type of {111} plane forms the guide groove. Hence, as is the side surface, the {111} plane is also formed at the end portion of the guide groove, and since this end portion has a tapered shape protruding toward the lens 35, there has been a limitation on decrease in distance between the LD 32 and the lens 35. For example, when the lens 35 having an outside diameter of 1,000 μm is used, the distance between the LD 32 and the lens 35 is increased to approximately 350 μm, and as a result, the optical coupling efficiency between the LD 32 and the lens 35 is extremely decreased.
In the optical coupling device described above, as a technique in which the distance between the LD 32 and the lens 35 is decreased, a mechanical machining technique has been disclosed in which the {111} plane 34 at the end portion of the guide groove 33 is ground in an approximately vertical direction by dicing. In addition, as a chemical wet etching method, a technique has been well known in which an end surface recessed in the optical axis direction is formed so as to have a V-shape by using a gallium arsenide (GaAs) substrate having two types of {100} planes. In this case, at the end portion of the guide groove, two types of {100} planes 39a and 39b, which are indicated by a two-dot chain line in FIG. 8, appear. According to the techniques described above, the end portion of the lens 35 can be disposed close to the LD 32, and the techniques are effectively used for improving the optical coupling efficiency.
However, the proposed techniques described above have the following problems. In the former technique, which is the mechanical machining technique, it has been difficult to perform alignment with sufficient accuracy. In addition, since the groove formed by dicing is below the bottom surface of the guide groove, the mechanical strength is decreased, and as a result, breakage may frequently occur in a subsequent element assembly step. Furthermore, since the machining can only be performed in the direction so as to traverse the substrate, part other than the guide groove is also ground, and as a result, formation of higher integration of elements on the silicon substrate will be considerably restricted thereby. On the other hand, according to the latter technique, which is the technique using a GaAs substrate, machining may be performed in a manner approximately equivalent to that for a silicon substrate; however, since a GaAs substrate is very brittle as compared to the silicon substrate, as is the case of the mechanical machining technique described above, breakage may frequently occur in an element assembly step.